Nitrate alleviates ammonium toxicity in Brassica napus by coordinating rhizosphere and cell pH and ammonium assimilation

In natural and agricultural situations, ammonium (NH4+) is a preferred nitrogen (N) source for plants, but excessive amounts can be hazardous to them, known as NH4+ toxicity. Nitrate (NO3-) has long been recognized to reduce NH4+ toxicity. However, little is known about Brassica napus, a major oil crop that is sensitive to high NH4+. Here, we found that NO3- can mitigate NH4+ toxicity by balancing rhizosphere and intracellular pH and accelerating ammonium assimilation in B. napus. NO3- increased the uptake of NO3- and NH4+ under high NH4+ circumstances by triggering the expression of NO3- and NH4+ transporters, while NO3- and H+ efflux from the cytoplasm to the apoplast was enhanced by promoting the expression of NO3- efflux transporters and genes encoding plasma membrane H+-ATPase. In addition, NO3- increased pH in the cytosol, vacuole, and rhizosphere, and down-regulated genes induced by acid stress. Root glutamine synthetase (GS) activity was elevated by NO(3)3- under high NH4+ conditions to enhance the assimilation of NH4+ into amino acids, thereby reducing NH4+ accumulation and translocation to shoot in rapeseed. In addition, root GS activity was highly dependent on the environmental pH. NO3- might induce metabolites involved in amino acid biosynthesis and malate metabolism in the tricarboxylic acid cycle, and inhibit phenylpropanoid metabolism to mitigate NH4+ toxicity. Collectively, our results indicate that NO3- balances both rhizosphere and intracellular pH via effective NO3- transmembrane cycling, accelerates NH4+ assimilation, and up-regulates malate metabolism to mitigate NH4+ toxicity in oilseed rape.